Solving the Uplift Puzzle

As building codes struggle to simplify prescriptive guidelines for wind-resistant structures, new measures for handling uplift emerge

Code Analysis

Any coastal builder knows that tough wind-related codes along the
Atlantic and Gulf seaboards have made structural design more
complicated. Where the codes once concentrated mostly on verifying
that buildings could handle gravity loads, now — ever since
Hurricane Andrew — coastal homes must also resolve the
complicated load effects associated with high winds: uplift, shear
(or "racking"), sliding, and overturning (Figure 1).

Figure 1. Engineers analyze the interactive
effects of high winds on a building in terms of four types of
building responses: (1) "uplift," the suction of wind on the roof;
(2) "shear" or "racking," the tendency of rectangular building
elements to deform into out-of-square parallelograms; (3)
"sliding," a sideways pushing force; and (4) "overturning," lifting
and toppling of the structure from the windward side. An engineered
design has to provide for connections of predictable strength to
resist all of these assumed load conditions.

In wind-speed zones up to 100 mph, the prescriptive requirements of
the 2006 International Residential Code (IRC) are sufficient for
wind-resistant design of most houses. In higher wind-speed zones,
however, rules call for an engineer to specify the structural
details — a process that may require a full, highly technical
analysis of the wind loads on the specific structure. Typically,
the resulting structural design will account for all the loads
using a system of engineered shear walls and diaphragms —
ignoring the possible strength contribution of other building
elements and relying heavily on metal hardware.

But that full engineering process can be time consuming and costly;
the hardware specified can be expensive; and for typical homes,
neither the case-specific analysis nor the structural connectors
are always necessary. As builders in coastal areas hit by Hurricane
Katrina struggle to rebuild affordable housing for an unprecedented
number of displaced residents, there is considerable pressure to
moderate the cost burden of wind-related design and construction
rules. To help reduce these costs, committees representing a
cross-section of the industry have been putting a concerted effort
into developing new codes, striving for cookbook-style guidelines
that builders and designers — not just engineers — can
understand and follow. In many cases, these new prescriptive
solutions can be accomplished with less reliance on expensive
hardware and more emphasis on ordinary wood framing and
sheathing.

The Uplift Load Path

Let's take a closer look at just one kind of loading: uplift. When
a high wind strikes a house, the flow of air over the roof creates
an upward suction, in the same way that wind creates lift on an
airplane wing. Wind tries to lift shingles and sheathing off the
roof, the roof sheathing pulls up on the roof rafters or trusses,
the roof structure pulls up on the wall plates (trying to lift the
walls), and the walls pull up on the floor deck or foundation that
they rest on. If any connection anywhere along that load path
fails, the structure can come apart. It's just one of the ways that
wind can destroy a house, but it's an important one and has to be
understood, designed for, and built for.

Engineers have spent countless hours designing hardware solutions
for strengthening the uplift load path (Figure 2). A typical
scenario looks like this: At the wall base, hold-downs and strap
anchors tie the sill to the foundation, and U-shaped or T-shaped
clips connect the wall plate to the studs. At the top of the wall,
the same clips connect studs to the top plate. Flat straps tie
lower-story wall studs to upper-story wall studs, bringing the
uplift load across the floor frame. And where walls meet roofs,
twist clips or other special straps tie down rafters and trusses.
All of these connectors have been thoroughly tested and rated for
their load capacity, so engineers have no trouble matching the
connector to the estimated design load. But for builders, each
metal connector adds cost and labor time.

Figure 2. In considering uplift, engineers
start with the upward pull on the roof sheathing and estimate the
uplift at each connection, including the rafter-to-wall joints, the
stud-to-plate joints, and the sill-to-foundation joints. The
integrity of the wall frame, acting as a unit against the upward
tensile pull, can be provided by rated steel connectors and nailed
sheathing, or a combination of the two.

Sheathing for uplift. Engineers working
for APA - The Engineered Wood Association, a plywood and OSB
industry group, have been pushing what they say is a less costly
alternative to metal hardware: full plywood or OSB wall sheathing,
carefully applied and heavily nailed in a way that lets the
sheathing and nails pick up much of the uplift load. If the
sheathing laps over the wall plates at top and bottom, and if the
panels are nailed to a framing member or blocking along all panel
edges, the sheathing and the nailed connection often have enough
strength to resist the uplift loads without any help from hardware.
Because coastal homes generally require structural wall sheathing
to resist racking, the panel industry argues, it only makes sense
to use the plywood or OSB to resist uplift as well, wherever
feasible.

Steel still required. In most situations,
steel framing hardware will still be required around window and
door openings (Figure 3) and at other places where special load
paths exist. Shear-wall elements typically still require hold-downs
as well. But for many clear wall sections, builders who fully
sheathe their walls may be able to skip the hardware (Figure 4). As
Louisiana building code consultant and longtime builder Paul
LaGrange points out, "It's easier when you can do something with
the sheathing instead of the metal connectors — easier,
faster, and cheaper."

Figure 3. Where plywood or OSB sheathing is
relied on to resist uplift, holes in the wall structure such as
window and door openings are a special case. Typically, load-rated
steel connectors will have to be used at the base of the window
jacks, at the points where headers rest on jack studs, and between
the headers and the framing above them. Design engineering is often
required, but a full wind-load analysis may not be necessary if
specifications can be found in a prescriptive guide such as the
Wood Frame Construction Manual (see "Resources,).

Figure 4. In two-story houses, uplift loads
from an upper story to a lower story have to be transmitted across
the second-floor frame. One solution to ensure successful
transmission is to connect steel straps from upper-story to
lower-story studs (above). Or, where the studs don't line up, to
connect studs from both stories into a band joist that is
structurally adequate to transfer the loading. Plywood or OSB
sheathing that spans across the floor system can also serve the
purpose (left), as long as all the edges are nailed to solid
framing or blocking. Guidance on sheathing-based techniques can be
found in APA Publication E510 (see "Resources").

Long sheets. The requirement for
continuous perimeter blocking and nailing of wall sheathing in
high-wind situations does involve some labor, so reducing the
number of sheathing joints can save time and expense. Glimpsing
opportunity, OSB manufacturer Norbord (www.norbord.com) is one
company that's developed a new line (www.windstormosb.com) of long
OSB sheathing panels for those who want to use sheathing to handle
uplift loads as well as building code braced-wall or shear-wall
requirements. These panels are just like any other 7/16- or
15/32-inch-thick OSB, except for the length: the company supplies
the material in 21 different lengths tailored to various wall
styles.

A Builder Adjusts

Louisiana's largest home builder, Southern Homes — one of the
state's few true "production" builders — switched to using
Norbord panels in early 2007. Says vice president David Stanton,
"Before that, we had U-straps on the bottom and top of the studs,
which are now gone. We had header straps on all window and door
headers; we've eliminated those too. And on the first floor of
two-story homes, we've eliminated corner hold-downs as well."

Uniform design. Southern Homes builds
primarily in the 110-, 120-, and 130-mph wind-speed zones.
Requirements in each zone are slightly different. But the company
uses a factory-built panelized wall system and relies on the same
two framing-trade contractors to frame houses in all its
neighborhoods. So for simplicity's sake, explains Stanton, "we've
chosen to build to the 130-mph wind speed in all the communities we
build in — whether it's actually below 110 mph or up to 130
mph — just to keep our framers and our fabricators straight."
Codes require a design professional to specify the wind-resistant
details in those high-wind areas. But Stanton notes, "With the
Norbord sheathing, we've eliminated so much strapping that it's
just not that big a deal on site."

The key detail is the nailing. "We're putting nails 3 inches
on-center on the edges of the panels and 6 inches on-center in the
field," says Stanton. Wall sections are delivered to the site fully
sheathed but with only one piece of the double top plate installed:
"So they are sending it out nailed in the first top plate at 3
inches on-center. Then our framer has to apply the second top plate
with the proper laps and nail that 3 inches on-center, too."

Tying in. The wall is not the whole
picture, of course — it has to be anchored into the
foundation below it and tied to the roof above it. Although
Southern Homes' outside engineer has not required hold-downs, he is
calling for closely spaced foundation anchors: "We're using 5/8- by
10-inch anchor bolts with 3-inch flat washers at 24 inches
on-center," explains Stanton, "plus we need two anchor bolts within
the first 8 inches from the wall corner."

Although wall sheathing can tie upper to lower stories adequately,
Southern Homes' wall-panelizing system doesn't allow for sheathing
to span between stories, notes Stanton: "The walls come out fully
sheathed, and then we put our floor truss system between the
stories and put sheathing on that." So to transmit the uplift load
between floors, crews still install straps between upper- and
lower-story wall studs. "We're putting straps on every stud across
the floors," says Stanton. And where roof trusses sit on wall
plates, every truss is tied down to the wall plate or to a wall
stud using a steel connector (Figure 5).

Figure 5. Plywood or OSB sheathing can resist
uplift loads within a wall structure as long as the sheathing fully
overlaps the plates and is sufficiently well nailed. But hardware
is still needed to tie the wall to the roof above it. Connectors
located on the outside plane of the wall line up with the sheathing
to create a load path of high capacity (far left). But if the
connectors simply tie the inside of the wall plate to the roof
framing, the plate can twist under a load, reducing the strength of
the connection (left). For that reason, truss or rafter ties
located to the inside of the wall should tie the roof directly to
the studs, not just to the wall plate.

Codes and Standards: Under
Construction

As Gulf Coast builders take on the challenge of rebuilding, code
and standard bodies are reworking the guidance they provide for
structural design of houses.

The main authority for engineered building design is ASCE-7 (the
American Society of Civil Engineers Standard 7), which lays out
highly technical procedures for estimating all the loads on
buildings, including wind loads. In specifying wood-frame solutions
to meet those loads, engineers can turn to the National Design
Specification for Wood Frame Construction, or NDS, published by the
American Wood Council (www.awc.org). Engineers draw from the NDS
their assumed values for the strength of sheathing nailed to
studs.

But analyzing the specific loads on a building based on its size
and shape, then adding up all the rated strengths of every nail in
every stud, plate, or joist is a tedious way to create a design. To
streamline the process for the bulk of homes — small, simple
buildings with standard details — the industry has come up
with prescriptive design manuals that apply to most ordinary
buildings. The American Wood Council publishes the Wood Frame
Construction Manual (WFCM), which has prescriptive solutions that
are "deemed to comply" with ASCE-7 for seismic, wind, and gravity
loads for a limited range of house sizes and shapes (see
"Resources," page 44). Even easier to use are the AWC's new "WFCM
guides" — five short handbooks geared to five different
wind-speed zones (90 mph, 100 mph, 110 mph, 120 mph, and 130 mph),
which provide drawings and tables specifying the main structural
elements of a wood-frame house in a high-wind area. Published in
cooperation with the International Code Council, the guides provide
a relatively quick and simple path to code compliance. For now at
least, says AWC engineer Jeffrey Stone, the council won't be
producing handbooks for 140-mph and higher wind zones: "Once you
get to 140 mph, 150 mph, in order to do a cookbook you're going to
be too conservative. It would be a lot easier just to go to the
Wood Frame Construction Manual and design it directly."

There's also a document called SSTD 10-99: "Standard for Hurricane
Resistant Construction," published in 1999 by the old Southern
Building Code Conference International (SBCCI), which is now part
of the International Code Council. SSTD 10-99, like the WFCM, has
cookbook answers appropriate for a limited range of typical,
moderate-size houses in hurricane wind zones. SSTD 10-99 is now
being revised and updated by an ICC committee and will eventually
be released as the ICC-600, "Standard for Residential Construction
in High Wind Regions," more or less in the same form as it already
exists.

Unfortunately, neither the WFCM guides nor the SSTD 10-99 standard
currently provide clear guidance on using wood structural panels to
resist uplift. According to Stone, descriptions of the method were
left out of those manuals because, while the theory based on the
load capacity of nails in wall sheathing is sound, there was no
laboratory research of full wall assemblies to verify the
assumptions. Now, however, testing at the NAHB Research Center and
at Clemson University has validated that OSB-sheathed wall sections
really do perform as predicted based on the nailed connection
values found in the NDS. So Stone, who serves on several of the
relevant code committees, says he expects to see prescriptive
guidance for sheathed walls that can handle shear loads as well as
uplift loads included in future editions of the AWC and ICC
standards.

In the meantime, says Stone, there's plenty of documentation now
available for builders to justify using wall sheathing to provide
resistance to wind uplift. But an engineer will still have to
specify the nailing ("usually it's double what you would need for
shear alone," explains Stone) as well as the foundation anchor
details and the roof tie-downs. In the case of shear walls, end
hold-downs may also be necessary to meet code.

And for Louisiana builders, there's specific interim guidance
already available: the Institute for Business & Home Safety's
2005 "Guidelines for Hurricane Resistant Residential Construction."
The IBHS guidelines are based on the SSTD 10-99 standard, but
include provisions for using plywood and OSB sheathing to resist
both shear and uplift in walls. The ICC-600 standard will include
very similar provisions when that document is finally released.
~

A former frame and finish carpenter, writer Ted
Cushman has been covering construction business and
technology since 1993.

Resources

The American Wood Council's Wood Frame Construction Manual (WFCM)
provides prescriptive solutions for standard houses that are
"deemed to comply" with ASCE-7 for seismic, wind, and gravity loads
(www.awc.org/Standards/wfcm.html).

Institute for Business & Home Safety's Guidelines for Hurricane
Resistant Residential Construction: Builders and design
professionals can request a free copy of the manual by emailing
info@ibhs.org or calling 866/657-4247.

SSTD 10-99: Standard for Hurricane Resistant Construction is
available for $34 (or less to members) from the International Code
Council (www.iccsafe.org/e/prodshow.html?

prodid=9058S99).

The APA - The Engineered Wood Association's technical note, Using
Wood Structural Panels for Combined Shear and Uplift, is available
at association's website (www.apawood.org). Search under
Publications, using "Form E510" or "uplift" in the publication
keyword search.